Happening of the condition starts the clock running for the purposes of adverse possession.

Fee simple on a condition subsequent

Clock starts when grantor exercises the right of entry.

Tacking?

Can tack periods of adverse possession, but the periods must pass directly from one adverse possessor to another, no gaps.
Can also tack periods of true ownership, meaning that the AP does not have to be against the present owner for the entire statutory period.

Disability?

Being a minor, insane, or in jail.

If owner is under disability at time AP starts possession...

the clock won't start to run until O is free of that preexisting disability.